Optical semiconductor devices in general, and Vertical Cavity Surface Emitting Lasers (VCSELs) in particular, have critical dimensions in their structures. Typically there will be several layers in such devices whose dimensions are related to the wavelength of the light at which they operate and that need to be grown to close tolerances.
In the case of photonic devices operating in the ultraviolet to green spectral region, the use of semiconductor materials in the InAlGaN material system is strongly favoured. Furthermore, high performance is favoured by growth of materials on gallium-rich group III nitride substrates or buffer layers. The shorter wavelengths of the light also mandate smaller semiconductor structures where greater control is required for the critical dimensions, especially in the vertical direction.
Many non-optical semiconductor devices also require great precision in the thickness of certain layers, especially as the geometries (the size of components and interconnections in the device) have shrunk.